Feds put Vermont Yankee uprate on hold due to excess vibration

BRATTLEBORO — Federal regulators have frozen the Vermont Yankee power increase at 105 percent after a measurement on Saturday recorded vibrations that exceeded acceptable levels, the Vermont Guardian has learned.

“The data forwarded to us on Saturday for the ‘A’ main steam line exceeded one of the criteria levels. So, in accordance with the monitoring plan, a hold has been placed on further power increases while the data is evaluated,” Nuclear Regulatory Commission spokesman Neil Sheehan told the Vermont Guardian in an e-mail late Monday.

The NRC last week issued approval to allow Vermont Yankee to increase power to 120 percent of its design capacity under close scrutiny because of concerns about the plant’s steam dryer, the component that removes water droplets from the steam before it feeds into the turbines.

The conditions require VY operators to increase power in increments of 5 percent and hold each increase for 96 hours after the vibration and stress measurements are sent to regulators.

“The NRC staff is independently evaluating the 105 percent data and will review the engineering evaluation [necessary for further power ascension] after it is completed by Entergy,” Sheehan said. “Our resident inspectors will continue to monitor Entergy’s actions onsite.”

An inspection of the VY dryer in November revealed more than 40 hairline cracks. VY officials said the fissures were probably old, and were detected with sophisticated magnification equipment first used during the most recent refueling outage to check 20 cracks found in the dryer in 2004.

Although the steam dryer is considered a non-safety component, experts say breakage could compromise the reactor’s safety systems if, for example, a piece of the cracked dryer were to break off and lodge in a valve.

Cracks discovered late last year in the welded reinforcements of the Dresden II reactor’s steam dryer in Illinois, which is similar to Vermont Yankee, also raised concerns at the NRC about the stability of the devices.

Dresden II, a boiling water reactor like Vermont Yankee, was shut down for a refueling outage when inspectors discovered fissures in six triangular stainless steel gussets that had been welded onto the plant’s cracked steam dryer in an effort to reinforce it.

“To NRC’s credit they’re saying let’s take a look at this,” said Ray Shadis, technical advisor to the anti-nuclear group New England Coalition. “But what we anticipate is that they will once again sharpen their pencils, do some calculations and figure out that maybe they can run a little bit longer.”

Last week Shadis said he didn’t expect VY to exhibit problems at 105 percent because operators last year told the NRC that they had already run the plant above 100 percent.

“If they have excessive vibrations or strain at 105 percent and the executives from VY have already admitted that they routinely run flow rates in excess of 100 percent, I have some concern that they should permit the reactor to run at all,” Shadis added.